The use of advanced composite structures has experienced tremendous growth in the aerospace, automotive and other commercial industries. Non-destructive evaluation (NDE) methods are often employed to detect inclusions, delaminations and porosities in an effort to ascertain the structural integrity of the composite structures. One method of NDE is laser ultrasound.
Laser ultrasound involves the use of lasers for generation and detection of ultrasound in materials such as composites. The technique offers the potential of rapid, non-contact inspection. Typically, a laser source produces ultrasonic surface displacements on the surface of a remote target. A second probe laser beam can detect the ultrasonic surface displacement on the surface of the remote target. Collection optics and instrumentation can then be used to process the probe laser beam and output data representing the ultrasonic surface displacements on the surface of the target.
FIG. 1 illustrates a conventional laser ultrasound inspection method. FIG. 1 employs pulse laser 10 to inspect an object, such as aircraft 16. Pulse laser 10 emits generation pulse 12 which is directed towards scanning mirror 14. Scanning mirror 14 steers generation pulse 12 to inspect aircraft 16. Various lenses for focusing may also be employed. A CO.sub.2 pulse laser may be employed as pulse laser 10. Pulse laser 10 deposits generation pulse 12 over a certain depth in an area on aircraft 16. Generation pulse 12 is converted to heat and causes expansion of an inspected area on aircraft 16. The expansion of the inspected area on aircraft 16 generates laser-generated ultrasonic waves.
The frequency content of the laser-generated ultrasonic waves contributes to the success of laser ultrasound as an NDE method. Higher frequencies yield better spatial resolution. However, for certain materials such as composites, high frequencies are attenuated more rapidly than lower frequencies. The thicker and more attenuative the component to be inspected, the lower the ultrasonic frequency content must be to avoid large attenuation. Consequently, with conventional methods the resolution and accuracy of defect detection tends to be limited for more attenuative materials.